A. Field of Invention
The present invention is related to wireless communication systems, and more particularly, to a method and system of measuring forward and reverse channel capacity in a wireless base station.
B. Description of Related Art
In a typical wireless communication system, an area is divided geographically into a number of cell sites, each defined by one or more radiation patterns created by an emission of radio frequency (RF) electromagnetic (EM) waves from a respective base transceiver station (BTS) antenna. Similarly, BTS antennae are configured for the reception of EM waves emanating from mobile devices. Each cell site is typically further divided into two, three, or more sectors, where the sectors provide transmit and receive radio coverage for a selected area within the cell site.
Each sector has a plurality of individual signal channels—both forward (from the BTS to the mobiles) and reverse (from the mobile to the BTS). Specifically, in CDMA communication systems, the individual communication channels are separable due to their use of channel-specific concatenated coding sequences. In the forward channels, a unique PN code (commonly referred to as a short PN code offset) is used to distinguish channels in a given sector from those in surrounding sectors and cells. Within each sector, channels are further distinguished by yet another code, termed a Walsh code. In an adjacent sector, the Walsh codes may be reused because channel separation is provided by a different offset of the short PN code for that sector. Thus, the number of available forward channels (BTS to MS) on a given carrier frequency in a sector is limited by the number of available Walsh codes. In the ANSI/TIA/EIA-95-B-99 standard entitled “Mobile Station-Base Station Compatibility Standard for Wideband Spread Spectrum Cellular Systems” (published Feb. 1, 1999), the contents of which are incorporated by reference herein, there are sixty-four available Walsh codes, while in CDMA 2000 series (TIA/EIA IS-2000 Series, Rev. A, published Mar. 1, 2000), one hundred twenty-eight Walsh codes are available. Both of the ANSI/TIA/EIA-95-B-99 and the TIA/EIA IS-2000 Series, Rev. A, standards are incorporated herein by reference, and are available from the Telecommunication Industry Association, 2500 Wilson Boulevard, Suite 300, Arlington, Va. 22201.
On the reverse channel, from the mobile to the BTS, a slightly different code concatenation is used. The Walsh codes are used to identify a data symbol alphabet, the short PN code is used for synchronization purposes, and the long code PN code is used to identify the individual mobile channel.
There are many factors that determine how many channels can coexist in a given cell or sector—inter-user interference being a significant one. That is, although the coding of the individual forward and reverse channels allows them to be distinguished from one another, each channel nonetheless causes a certain amount of interference with the other channels. For the forward channels, the cumulative interference of the traffic channels must not prevent the mobiles from receiving the pilot, sync and paging channels. Typically, about twenty-five percent of the transmitted power must be reserved for the pilot, sync and paging channels. For the reverse channels, the base station is receiving signal energy from all of the active mobiles. For the channel of interest, the other mobiles appear as interfering noise, which makes it appear as if the effective noise floor is rising. This effect, therefore, is referred to as “reverse noise rise”(RNR). The increased RNR causes the mobile to have to transmit at a higher power level, which in turn causes an increase in the RNR for the other mobiles. Thus, overall system capacity is related to how RNR is affected by additional mobile units.
Theoretical capacity limits have been developed to predict system capacity, such as the pole point equation. The pole point equation specifies the point at which an additional mobile would require an infinite amount of power, and for a six-sectored site for radio configuration 1 and EVRC, is given by:
      Pole_Point    =                  Processing_Gain                              6            SG                    *          VAF          *                      (                          1              +              INT                        )                    *                                    E              b                                      N              o                                          +      1        ,where Processing Gain=Bandwidth/Data Rate (1228800 Hz/9600 bits per sec, or 128); SG=Sectorization Gain=4.5; VAF=Voice Activity Factor=0.40; TNT=Adjacent Cell Interference=0.60; Eb/No=the ratio of the energy per bit to the orthogonal noise=6.2 dB=4.17. These values predict a pole point value of:
  Pole_Point  =                    128                              6            4.5                    *          0.40          *                      (                          1              +              0.60                        )                    *          4.17                    +      1        =          36.9      ⁢                          ⁢      users      
Thus, when designing a system to have a capacity of fifty percent of the pole point, a value of 18.4 users is predicted. However, actual system performance varies significantly from the performance predicted from theoretical models. Such models may not accurately account for the effects of, for example, newly developed power control algorithms, or particular geographic layouts of sectors, or the time-varying geographical distribution of users within the sector. As a result, actual measurements of cell/sector channel usage may be preferred.
Current techniques of data collection offered by BTS manufacturers is insufficient to obtain a complete understanding of the cell and sector loading, interferences levels, and other characteristics necessary to determine the true network performance. Typically, presently available systems provide small samplings of data, if any. Consequently, a capacity measurement system and method that overcomes the current limitations is needed.